This invention relates generally to semiconductor diode lasers and more particularly to semiconductor ridge waveguide diode laser exhibiting low divergence and single mode operation.
In numerous applications, it is desirable to have an output from a semiconductor laser to have a low divergence both in the lateral direction as well as the transverse direction. Typical uses for such low divergence semiconductor diode outputs include free space optical communication, fiber coupled optical communication, and use as an optical source for pumping solid state lasers. In prior semiconductor laser devices which have been designed to have a relatively high output power as well as a single mode output, the divergence of the output beam emitted from the emitting facet has been quite large. A typical far-field divergence of a single mode semiconductor device is typically 50.degree. in the transverse direction and 7.degree. in the lateral direction as shown in an article entitled, "High Power 0.98 .mu.m GaInAs Strained Quantum Well Lasers for ER.sub.3+ -Doped Fiber Amplifier" published in Electronics Letters, Volume 25, pages, 1563-1564 in 1989 by Akyasu, et al.
Various laser diode configurations comprised of semiconductor layers of GaAs/AlGaAs have been designed in order to produce relatively high power outputs with single mode operation. Such configurations have included the buried heterostructure laser, the distributed feedback laser, the distributed Bragg reflector laser, and the ridge waveguide laser. However, all such prior devices while achieving relatively high output power levels in single mode operation suffered from fairly large divergence angles. While Welsh, et al reported in an article entitled, "High Reliability, High Power, Single Mode Laser Diodes" in Electronics Letters, Volume 26, pages 1481-1483 in 1990 that they had been able to reduce transverse divergence to approximately 22.degree., significant reductions of the divergence of the laser's output in both the transverse and the lateral directions have not yet been accomplished.
The far-field divergence of a semiconductor laser diode is a measurement of the extent of spread of the light beam as the beam moves away from the emitting facet of the laser into free space. The far-field divergence is measured in two directions at the full width half maximum of the radiation pattern and is expressed in degrees. The full width half maximum of the radiation pattern is a measurement taken, in the far-field in this instance, of the total angular divergence between points in space at which the power level of the radiated light is one half the maximum value radiated in that plane in space. The full width half maximum of the radiation pattern is measured in two directions both perpendicular (or transverse) to the PN junction of the semiconductor device and parallel (or lateral) with the PN junction. A reduction in the far-field divergence is desirable as such a reduction would enable more emitted light to be collected by a lens system or an optical fiber as the radiated light would be less diverse. Thus, by allowing more light to be collected by a lens system, or an optical fiber, the efficiency of an optical communications system can be greatly improved as less emitted light from the semiconductor laser diode would be lost.
Therefore, it would be desirable for a semiconductor diode laser to produce a high power, single mode output while achieving low divergence. It would furthermore be desirable that such divergence be decreased in both the transverse and lateral directions in the far-field.